1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a grid having an improved arrangement of multiple dimples for supporting nuclear fuel rods.
2. Description of the Prior Art
In most nuclear recactors, the reactor core is comprosed of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control rod guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods.
The grids as well known in the art are used to precisely maintain the spacing between the fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally retain the rods against longitudinal movement. Conventional designs of grids include a multiplicity of interleaved straps having an egg-crate configuration designed to form cells which individually accept the fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fuel rods at a given axial location therealong typically use relatively resilient springs and/or relatively rigid protrusions (called dimples) formed into the metal of the interleaved straps. The springs and dimples of each grid cell frictionally engage or contact the respective fuel rod extending through the cell. Additionally, outer straps are attached together and peripherally enclose the grid straps to impart strength and rigidity to the grid.
Representative of the prior art are the grids described and illustrated in U.S. Patents to Andrews et al (No. Re. 28,079), Warberg (No. 3,679,547), Masetti (No. 4,090,918), Laird et al (No. 4,111,348), Jabsen (Nos. 4,124,444 and 4,125,435), Walters (No. 4,137,125), Wohlsen (No. 4,325,786), Feutrel (No. 4,364,902), Leclercq et al (No. 4,411,862), and Hellman et al (No. 4,474,730).
For grids fabricated of zircaloy type materials, grid springs are not able to maintain their design preload after installation of the fuel assembly into the nuclear reactor core due to irradiation and temperature effects. In some circumstances it is difficult to achieve the spring rates and spring forces considered necessary to control bowing of fuel rods.
Consequently, a need exists for an improved dimple arrangement in fuel assembly grids which will provide the force necessary to support the fuel rods in the reactor core with minimal degradation over time.